Question

I have 2 machines running a simple C TCP server that I have written for testing purposes, 1 with Fedora 16, the other with Ubuntu 11.10. My Fedora machine works perfectly but on the Ubuntu machine, recv() does not block. Please keep in mind that these machines are running the same exact code. Has anybody seen this before? Thanks

int TcpSocket::ReadFromClient(int socket, char* buf, int len)
{
    char *request = buf;
    int slen = len;

    int c = recv(socket, request, slen, 0);
    while((c > 0) && (request[c-1] != '\n'))
    {
        request += c;
        slen -= c;
        c = recv(socket, request, slen, 0);
    }

    if (c < 0)
    {
        return c;
    }
    else if(c == 0)
    {
        //Sending back an empty string
        buf[0] = '\0';
    }

    return len-slen;
}

Answer 1

It looks like the intention of your code is to stop reading when a '\n' byte arrives. If that is the case then you need to read from the socket 1 byte at a time instead of using the entire available buffer size, especially since you are only checking the last byte of the buffer instead of checking every byte received.

You should also change the loop logic to only call recv() in one place instead of two places. Your current implementation is calling recv() with slen=0 when the buffer is exhausted, which will set c=0 and nullify the first byte in the buffer.

Try this instead:

int TcpSocket::ReadFromClient(int socket, char* buf, int len)
{ 
    int slen = len;
    char ch;

    while (len > 0)
    {
        int ret = recv(socket, &ch, 1, 0); 
        if (ret > 0)
        {
            *buf = ch; 
            ++buf; 
            --len; 

            if (ch == '\n')
                break;
        }
        else
        {
            if ((ret == 0) || (errno != EAGAIN))
                return ret;

            fd_set readfd;
            FD_ZERO(&readfd);
            FD_SET(socket, &readfd);

            timeval tv;
            tv.tv_sec = 5;
            tv.tv_usec = 0;

            ret = select(socket+1, &readfd, NULL, NULL, &tv);
            if (ret < 0)
                return ret;

            if (ret == 0)
            {
                // timeout elapsed while waiting for data
                // do something if desired...
            }
        } 
    } 

    return slen - len;
}